I'm a contractor for two reasons: (1) money and (2) because I don't buy into the corporate PR. People who work in these places have to buy into the corporate values system.
I look at corporate values with great skepticism; perhaps I'm just cynical but in big companies it always seems to end up (in performance reviews) as being a way for the company to come back and justify why your bonus, increase, etc. will fall short of expectations this year.